Uncaught TypeError: x.attr is not a function [duplicate]

so in my HTML i got this:

<li data-target="#multi-item-example" data-slide-to="2" class="m-0 mr-2 active"></li>

my JS makes this:

var x = document.getElementsByClassName("m-0 mr-2 active");
var y= x.getAttribute("data-slide-to");
alert(y);

but I get:

Uncaught TypeError: x.getAttribute is not a function

want to get the value of data-slide-to with as little code as possible.

This works but is way too long:

var nodes=[], values=[];
for (var att, i = 0, atts = x.attributes, n = atts.length; i < n; i++){
    att = atts[i];
    nodes.push(att.nodeName);
    values.push(att.nodeValue);
}